Date: Mon, 4 Sep 2017 20:29:32 +0100 From: Ard Biesheuvel <ard.biesheuvel@...aro.org> To: Nicolas Pitre <nicolas.pitre@...aro.org> Cc: "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Arnd Bergmann <arnd@...db.de>, Russell King <linux@...linux.org.uk>, Kees Cook <keescook@...omium.org>, Thomas Garnier <thgarnie@...gle.com>, Marc Zyngier <marc.zyngier@....com>, Mark Rutland <mark.rutland@....com>, Tony Lindgren <tony@...mide.com>, Matt Fleming <matt@...eblueprint.co.uk>, Dave Martin <dave.martin@....com> Subject: Re: [PATCH v2 24/29] ARM: kernel: implement randomization of the kernel load address On 4 September 2017 at 19:44, Nicolas Pitre <nicolas.pitre@...aro.org> wrote: > On Sun, 3 Sep 2017, Ard Biesheuvel wrote: > >> This implements randomization of the placement of the kernel image >> inside the lowmem region. It is intended to work together with the >> decompressor to place the kernel at an offset in physical memory >> that is a multiple of 2 MB, and to take the same offset into account >> when creating the virtual mapping. >> >> This uses runtime relocation of the kernel built as a PIE binary, to >> fix up all absolute symbol references to refer to their runtime virtual >> address. The physical-to-virtual mapping remains unchanged. >> >> In order to allow the decompressor to hand over to the core kernel >> without making assumptions that are not guaranteed to hold when >> invoking the core kernel directly using bootloaders that are not >> KASLR aware, the KASLR offset is expected to be placed in r3 when >> entering the kernel 4 bytes past the entry point, skipping the first >> instruction. >> >> Cc: Russell King <linux@...linux.org.uk> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org> >> --- >> arch/arm/Kconfig | 15 +++ >> arch/arm/kernel/head.S | 103 ++++++++++++++++++-- >> 2 files changed, 109 insertions(+), 9 deletions(-) >> >> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig >> index 300add3b8023..fe4a2cd1f15c 100644 >> --- a/arch/arm/Kconfig >> +++ b/arch/arm/Kconfig >> @@ -1825,6 +1825,21 @@ config XEN >> help >> Say Y if you want to run Linux in a Virtual Machine on Xen on ARM. >> >> +config RANDOMIZE_BASE >> + bool "Randomize the address of the kernel image" >> + depends on MMU && AUTO_ZRELADDR >> + depends on !XIP_KERNEL && !ZBOOT_ROM > > This should work even if ZBOOT_ROM is selected. The ZBOOT_ROM option > allows for the decompressor executing directly from ROM and > decompressing the kernel anywhere in RAM. > In principle, yes. But currently, the code is not entirely XIP clean, i.e., the kaslr_offset and __efi_boot variables are in .text but assumed to be writable. For UEFI, this does not really matter, because that implies !XIP. However, I agree it would be nice if ZBOOT_ROM could run this code as well, but it requires a global variable. I will try to make that work.
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